Buggy code:
public abstract class Page extends MarkupContainer implements IRedirectListener , IPageMapEntry { private static final long serialVersionUID = 1L ; public static final int LATEST_VERSION = - 1 ; private static final short FLAG_IS_RENDERING = FLAG_RESERVED2 ; private static final short FLAG_NEW_VERSION = FLAG_RESERVED3 ; private static final short FLAG_TRACK_CHANGES = FLAG_RESERVED4 ; private static final Log log = LogFactory . getLog ( Page . class ) ; private short autoIndex ; private FeedbackMessages feedbackMessages ; private short numericId ; private transient PageMap pageMap ; private String pageMapName ; private transient Set renderedComponents ; private transient boolean stateless = true ; private IPageVersionManager versionManager ; protected Page ( ) { } protected Page ( final IModel model ) { } protected Page ( final PageMap pageMap ) { } protected Page ( final PageMap pageMap , final IModel model ) { } public void afterCallComponent ( final Component component , final RequestListenerInterface listener ) { } public void beforeCallComponent ( final Component component , final RequestListenerInterface listener ) { } public final boolean checkAccess ( ) { } public void detachModels ( ) { } public final void dirty ( ) { } public final void doRender ( ) { } public final void endComponentRender ( Component component ) { } public final void expireOldestVersion ( ) { } public final short getAutoIndex ( ) { } public final int getCurrentVersionNumber ( ) { } public final FeedbackMessages getFeedbackMessages ( ) { } public final String getId ( ) { } public int getNumericId ( ) { } public final Class getPageClass ( ) { } public final PageMap getPageMap ( ) { } public IPageMapEntry getPageMapEntry ( ) { } public final long getSizeInBytes ( ) { } public Page getVersion ( final int versionNumber ) { } public final int getVersions ( ) { } public final String hierarchyAsString ( ) { } public boolean isErrorPage ( ) { } public final void onRedirect ( ) { } public final void removePersistedFormData ( final Class formClass , final boolean disablePersistence ) { } public final void setNumericId ( final int id ) { } public final void startComponentRender ( Component component ) { } public String toString ( ) { } protected void configureResponse ( ) { } protected final void internalOnEndRequest ( ) { } protected final void internalOnModelChanged ( ) { } protected IPageVersionManager newVersionManager ( ) { } protected void onRender ( final MarkupStream markupStream ) { } final void componentAdded ( final Component component ) { } final void componentModelChanging ( final Component component ) { } final void componentRemoved ( final Component component ) { } final void componentRendered ( final Component component ) { } final void componentStateChanging ( final Component component , Change change ) { } final boolean isStateless ( ) { } final void setStateless ( boolean stateless ) { } final void setFormComponentValuesFromCookies ( ) { } final void setPageMap ( final PageMap pageMap ) { } private void checkHierarchyChange ( Component component ) { } private final void checkRendering ( final MarkupContainer renderedContainer ) { final IDebugSettings debugSettings = Application . get ( ) . getDebugSettings ( ) ; if ( ( debugSettings . getComponentUseCheck ( ) ) && ( ! ( getResponse ( ) . isRedirect ( ) ) ) ) { final Count unrenderedComponents = new Count ( ) ; final List unrenderedAutoComponents = new ArrayList ( ) ; final StringBuffer buffer = new StringBuffer ( ) ; renderedContainer . visitChildren ( new IVisitor ( ) { public Object component ( final Component component ) { if ( ( ( renderedComponents ) == null ) || ( ! ( renderedComponents . contains ( component ) ) ) ) { if ( component . isAuto ( ) ) { unrenderedAutoComponents . add ( component ) ; } else if ( component . isVisibleInHierarchy ( ) ) { unrenderedComponents . increment ( ) ; buffer . append ( ( ( ( ( Integer . toString ( unrenderedComponents . getCount ( ) ) ) + ".<seq2seq4repair_space>" ) + component ) + "\n" ) ) ; } } return CONTINUE_TRAVERSAL ; } } ) ; for ( int i = 0 ; i < ( unrenderedAutoComponents . size ( ) ) ; i ++ ) { ( ( Component ) ( unrenderedAutoComponents . get ( i ) ) ) . remove ( ) ; } if ( ( unrenderedComponents . getCount ( ) ) > 0 ) { renderedComponents = null ; <START_BUG> throw new WicketRuntimeException ( ( "The<seq2seq4repair_space>component(s)<seq2seq4repair_space>below<seq2seq4repair_space>failed<seq2seq4repair_space>to<seq2seq4repair_space>render:\n\n" + ( buffer . toString ( ) ) ) ) ; <END_BUG> } } renderedComponents = null ; } private final void endVersion ( ) { } private final void init ( ) { } private final void init ( final PageMap pageMap ) { } private final boolean mayTrackChangesFor ( final Component component ) { } }

Fix:
throw new WicketRuntimeException ( ( "The<seq2seq4repair_space>component(s)<seq2seq4repair_space>below<seq2seq4repair_space>failed<seq2seq4repair_space>to<seq2seq4repair_space>render.<seq2seq4repair_space>A<seq2seq4repair_space>common<seq2seq4repair_space>problem<seq2seq4repair_space>is<seq2seq4repair_space>that<seq2seq4repair_space>you<seq2seq4repair_space>have<seq2seq4repair_space>added<seq2seq4repair_space>a<seq2seq4repair_space>component<seq2seq4repair_space>in<seq2seq4repair_space>code<seq2seq4repair_space>but<seq2seq4repair_space>forgot<seq2seq4repair_space>to<seq2seq4repair_space>reference<seq2seq4repair_space>it<seq2seq4repair_space>in<seq2seq4repair_space>the<seq2seq4repair_space>markup<seq2seq4repair_space>(thus<seq2seq4repair_space>the<seq2seq4repair_space>component<seq2seq4repair_space>will<seq2seq4repair_space>never<seq2seq4repair_space>be<seq2seq4repair_space>rendered).\n\n" + ( buffer . toString ( ) ) ) ) ;